1. Field of the Invention
The invention concerns a reduction gear system adapted to drive the screw shafts in twin screw extruding machines.
2. Description of the Prior Art
Single screw extruding machines are routinely used for extruding plastics materials and many compounds of varying complexity based on polymers, although it is well established that there are clear advantages in the use of twin screw extruding machines. In particular, the throughput of a twin screw extruding machine is substantially proportional to the screw rotation speed, whereas that of a single screw extruding machine reaches relatively rapidly a maximum value after which it descreases. Also, the power required for extrusion using a single screw machine is proportional to the square of the mass of material extruded, whereas it has a lower value for the same throughput in the case of a twin screw machine.
Twin screw extruding machines are not common, in spite of their functional superiority, because of difficulties experienced with driving them in rotation, due to the small distance separating the axes of the two screws. The torque to be transmitted is very high and the size of the pinions which can be installed in the space available provides for only one meshing. This results in high bending loads on these pinions of limited dimensions and in high reaction forces on the bearings supporting the shafts of these pinions.
It is not possible to use a single bearing capable of withstanding these reaction forces. Instead a number of needle bearings are placed side by side in the hope that the reaction forces will be equally distributed between them. However, the bending of the shaft creates irregular loads which result in premature deterioration of the bearings.
By virtue of the extrusion process the screws generate a high axial thrust which must be countered by thrust bearings which must be relatively small to fit into the space available for them, but of adequate dimensions to withstand the load during the theoretical service life of the machine.
The conventional method currently used to counter the reaction to the axial thrust of the screws consists in mounting an appropriate thrust bearing of large diameter at one end of one screw and associating with the second screw a number (up to four) of consecutive thrust bearings of small diameter, in the space available between the two screws. These successive thrust bearings comprise flexible means which it is hoped will automatically distribute the thrust between the thrust bearings in an appropriate manner. Practice shows that one of them is subject to an exaggerated thrust and deteriorates rapidly.
The principal object of the invention is to provide a reaction gear system for twin screw extruding machines with which the power which may be transmitted, and therefore the torque, may be doubled, the risk of bending may be eliminated and the load on the bearings may be reduced to zero.
It should be pointed out here that an additional constraint is involved in driving twin screw extruding machines. The screws may turn in the same direction or in opposite directions; it is therefore necessary for any reduction gear system for twin screw extruding machines to be readily adaptable to the simultaneous driving in rotation of two screws in the same direction or in opposite directions.
Another object of the invention is to provide a reduction gear system which can be readily adapted to co-rotating or counter-rotating screws, using a common casing.